CGEM Problem - Worm Adjustment

Here's the setup...

I took my CGEM out 2 nights ago, Tuesday night, and I experienced trouble. When slewing in Declination, the mount was making some wierd noises. Forgive me for this... Normally, the mount just made the familiar "rrreeeeeeeeeeeeee" noise when slewing. Mine was going "rrrreeeeeeeeerrrrruuuuuuuuuurrrrrreeeeeeeeerrrrruuuuurrrr". It's hard to describe a sound in words, but it sounded like the mount was binding or at least the drive was encountering alternating areas of increasing and decreasing resistance. Coupled with this was the inability to autoguide reliably.

Wednesday, I removed the control board from the side of the mount and pulled off the front plug for the polar scope. I looked around as best I could at the limited portion of the gear train that's visible. Nothing looked out of normal. I decided to head outside again for further investigation. The same stuff was going on, but I actually managed some good subs of M8 (posted in the DSLR forum). My guiding was still failing every so often, and the declination distance was going off the graph in PHD. Here's a shot of what PHD was showing.



I pulled the plug early last night, and this morning, I did some more investigation. I found that when moving the declination axis by hand, there were in fact 'sticky' areas.

I searched around the CGEM Yahoo group, and I found information posted by a "tmwaits1" about worm gear slop. I'm not sure if this gentlemen is a member here, but if he is, I want to extend a huge thank you for the information you have made available on the CGEM group.

I got to thinking that I have the opposite problem going on. I don't have slop, but I have binding. I decided to make adjustments to the worm gear slop. After doing so, I don't think that my mount has ever moved more freely by hand (with the clutches disengaged) and it has never been quiter.

Here's a little rundown of what I did...

Declination Adjustment

First, loosen the Declination Clutch

Next, you have to slightly loosen the four 5mm hex bolts that hold the worm block.

Left side - 2 bolts (control panel side)


Right side - 2 bolts


Next, you have to alternately adjust 2 2mm hex bolts to move the worm gear closer to or further away from engagement.

Tightening the top 2mm hex will bring the worm into more positive engagement and will reduce slop. If overdone, you can also create binding (my situation).


Tightening the bottom 2mm hex will pull the worm away from engagement, increasing slop, but relieving binding.



That's it, just tighten the 4 5mm hex bolts back up and you're good to go. With the Declination clutch tightened, you should have minimal slop and no binding. It make take a little back and forth through this to get you where you want to be. During the adjustment, I would tighten the clutch and check slop, then loosen the clutch and feel for any stiction. Just find the happy medium.

RA Adjustment

This is the same deal as Declination, but with different bolts.

RA Left side - 2 bolts (control panel side)


RA Right side - 2 bolts


RA 2mm hex - front - tightening this will pull the worm into more positive engagement and decrease slop.


RA 2mm hex - rear - tightening this will move the worm away from engagement and increase slop, but relieve stiction


Be careful that two of the 2mm hex bolts for the RA and Declination worms are right by eachother on the back side of the mount. The top one is RA, and the bottom is Declination. Just pointing that out to make sure the pictures above are clear.

Anyway, I thought I would post this info here becasue it may help someone else in the future. I'm not sure how my mount got to the sticking point, because I've never adjusted anything in the past, but never the less, it did.

I'm pretty confident that this is going to solve the problem I was having, but the true test will be another imaging run. I've got some random clouds around now, and if I said I was going to spend another night in the back yard, you'd all probably wind up speaking about me in the past tense. In lieu of that, it will be a little while until I am able to fully retest this.

Adam,

Thanks for the post and great pictures and explanation. My problem with my CGEM is similar but only in the RA axis. You probably saw my posts in the group about this and John's posts. He shipped his mount back to Celestron for checkout / adjustment but I took another route. I've shipped mine off to be HyperTuned with Teflon and ceramic hybrid upgrades. I'll post in a separate thread once it comes back and I get a chance to test it out.

Hope your adjustments solve your problem! There is such a lack of info online about actually doing any self-service on the CGEM. Much of what I've read and heard say it's the same as the Atlas, but hit me with a stick, I just don't see it. They ARE different enough physically and electronically that I've been very hesitant about doing any adjustments..... Good luck!

Hi George,

Hopefully your mount comes back soon. Everything I have read about Hypertuning has been positive. I'm sure you'll be happy with it.

I totally agree that there is almost no information to be had anywhere about CGEM adjustments. In fact, the post on the CGEM group that I mentioned before actually contained pictures of an EQ6 mount. The basic principle of the adjustment is the same, but some of the bolts are placed differently.

Ultimately, I intend to get myself inside an observatory on my property, and when that happens, I plan on upgrading my mount. If it weren't for that long-range goal, I would have probably sent my CGEM for Hypertuning. As it is, I took matters into my own hands. We'll see how it goes. I have a little travel over the weekend and the beginning of next week, but hopefully I'll have another night of imaging by the end of next weekend.

Just curious, but why isn't Celestron ( with all their 50 year anniversary hype ) catching these before they get in your hands?

I was considering a CGEM, but I don't think I will buy one now. This is yet again another problem I have head of with this mount. I am really curious why there are such stark differences between the Orion Atlas EQ-G and the CGEM when both are produced by Synta. Makes me think Celestron is telling them to cut corners.

My mount has been in my posession for a little over a year, but I am not the first owner. The mount did not have the problem outlined above until recently, and I'm not sure why the problem reared its head.

In all honesty, I don't fault Celestron or the mount. I don't think it is unreasonable for the worm gears to need adjusting after years of service.

Atlas / EQ6 owners have also had the problem I described. It seems to be fairly normal to make worm adjustments over time.

Agreed. The web is full of links about adjusting and hypertuning GEM mounts, all classes, all types. In fact, it's expected that these adjustments will have to made hence the adjustment screws in the first place. One thing to keep in mind, there are a ton of Atlas and CGEM mounts in use. Most of them perform well, with a few exceptions... but what you don't see is alot of them for sale. Why? If, in the case of the CGEM, it is a problem mount, riddled with problems, why are there none currently listed here or on AstroMart for sale? at least the quick look I just did ... will this change as these mounts age and wear? Of course ... but as in the case of the Atlas, refurbishing the mount with new grease, bearings, etc... gives these mounts a new lease on life. Also, keep in mind that the HC firmware for the CGEM is one of its key features. Celestron has been improving this firmware over the past 18 months or so and the lastest release allows you to change the default slew and goto DEC / RA rates. .... just my 2 Eventually I will upgrade, probably to a Tak Em200, but for now, I'll hang on to my CGEM, upgraded yes, but I am honestly looking to use it for another 2 or 3 years. Good luck.

Thanks for the pictures Adam, most interesting. Question, how fragile are the 5mm and 2mm hex bolts for adjusting the worm? A 2mm bolt seems awfully small for the size of the mount, are they prone to stripping after a short time? That would really stuff up the works if that happened...? Fox.

Sorry, I shouldn't have said 2mm "hex bolt". The 2mm's act as a set screw. There's not really any loading on the threads when making the adjustment. Once the 5mm's are loose, the two 2mm set screws are just setting up position of the worm.

Even with all the adjustments to an EQ mount regardless of being manual or motor driven, I have found the slop to be a problem only when the scope's RA transverses over zenith. This is just an inherent result of design vs. gravity, until it is improved with an alternate mechanism, us EQ users will have slop of some degree. For clock drives, such as mine, I just back up the scope a bit to compensate so it is tight again for tracking after it rolls over the zenith. Playing with the counter weight adjustments to get the scope balanced is certainly helpful before going at adjusting the worm gear. Too tight and it may interfere with the drive motor and ruin it. In any case, we all have a solution that is sound and unique to each situation! Good luck & Clear Skies all !

Adam,
I just wanted to thank you for posting this, I have filed your procedure and pictures away for future use. Good luck with the new house and darker skies (yes I read about it in the AP forums) I would actually be jealous of either location, but the new one looks awesome!
Cheers!

Thanks Kirk, and no need to thank me for posting up pictures and such. I'm new to GEMS (about a year with the CGEM) and I'm learning and working things out. If my experiences can help others, then that makes me happy. I have an engineering background, and I'm not afraid to tinker, so in some ways, I make a good guinea pig.

Adam,

Thank you, I just got my CGEM and it had a little bit of slop, 10 minutes tightening the gears fixed it. YOU ARE THE MAN!!!!!

Thanks

No problem. I'm pretty new at all this, but if sharing my experiences helps someone else, then I'm happy.

Your article was helpful in that my CGEM with 11in Nexstar was giving me fitts when ever I touch the focus knob, sure I have to use my hands and there is always some vibration but tht few nights there was a wobble left to right in dec and you could see your target go from one side of the field of view to the other, very irritating.
When I removed the scope and checked close you could grab either the dovetail clamp and it had side to side play in declination and It did the same in ra. Not much in fact so little you could feel it but not really see it.
Using your instructions I was able to correct the isssue with a slight adjustment on the worm gears of both axis, and yes I did have to back off some after the first try as it was creating a strain on the drive and you can hear it but I was lucky to get the play completely out and end up with smooth opeeration of both dec & ra movement.
I agree it seems to me that is some info that Celestron should have included in the manual with the CGEM or at least on their site.
ALL HAIL CLOUDY NIGHTS- where the truth comes out sooner or later

This is a gret post. I have notice a problem with my CGEM also sounding like it is binding when slewing to certain positions. It may be a balance problem, but this is great info if needed.
Thanks for the info.

Atlas / EQ6 owners have also had the problem I described. It seems to be fairly normal to make worm adjustments over time.

I've adjusted mine. Same approach as the CGEM.

I find it funny and bizarre that you set worm mesh using a 2mm hex screw that moves the ENTIRE casing... but that's just me (it's the same on the Atlas I just find it a weird, awkward and difficult to get right design.

Check your worm end float as well, that was loose on my Atlas and was causing me alot of phantom issues that would come and go.

Simple.

The cost of doing additional QC before shipment is more than the cost of dealing with support requests and returns later.

Regards,

Jim

Hi Adam,

if you want a look at the insides, I've posted a bunch of pictures I took during my HyperTune on the CGEM Yahoo site. Look for both "John W.'s CGEM stuff" and "RA Parts" in the Photos section.

Adam E, thank you so much for posting this. I was starting to get pretty upset that my very expensive mount was fast becoming useless.

It was a satisfying project, but I did have some confusion for a while... The instructions for reducing wobble in RA is what finally worked for getting rid of wobble in DEC, took me a while to figure that out.

Besides engaging and disengaging the clutch, I'd suggest periodically turning the motor on and listening to the sound. t one point I had the hex too tight and my RA couldn't even move.

It was unclear to me if I had to tighten the restraining hexes before doing these tests or not, what do you think?

On my CGEM and the other one I have been using the master gear wheels are very obviously slightly eccentric. If the worm mesh is perfect where the slop is worst, then the motors very obviously struggle on the tightest part of the gear. With that said, this appears to be in the DEC axis only, so it is conceivable they have a lower tolerance on that axis. Now that it is out of warranty, doing something like swapping out for Aeroquest gear sets wouldn't be so scary, but with it adjusted for the eccentricity, the mount seems to work perfectly, even with a C11 on it.

-Rich

Can someone post a link to where we can find a tear down of the CGEM? I would like to take a look at the gears, and adjust. It's still under warranty but I really don't want to pay $100+ for shipping both ways. I've got binding, I am using the tips above, but would still like to know more about the inner working of this mount, Thanks..

Here is a video you could check out. Or this flickr page. I am not sure if you want to mess with the mount while it is under warranty. YMMV! Good luck

Thanks nemo129, I saw a vid on you tube where some guy was making adjustments to the gears, he had like 4 allen head screws he loosened just a bit then was moving the gears back and forth to get the perfect mesh between them, I kinda wanted to know how to tear it down to that point? He didn't show that part, and I don't want to mess things up to bad   Thank you for the links!! Maybe I'll just send it in, after the rains hit that is

 

This is the vid I mentioned, question still remains how to tear it down to that point? I know it's a VX mount, but figured it would be pretty much the same..

 

http://youtu.be/ORGleQBhkLw 

Edited by DonDon, 20 September 2014 - 05:06 PM.

Well I found out the 2 screws on the right side show the gears, that might be a big help when trying to adjust, thanks again for the links, very helpful.